1. Field of the Invention
The present invention relates to a light-emitting element, a display device, a light-emitting device, an electronic device, and a lighting device each of which includes an organic compound as a light-emitting substance.
2. Description of the Related Art
Advances are being made in application of a current excitation type light-emitting element in which an organic compound is used as a light-emitting substance, i.e., an organic EL element, to light sources, lighting, displays, and the like.
As is known, in an organic EL element, the generation ratio of excitons in a singlet excited state to excitons in a triplet excited state is 1:3. Thus, the limit value of internal quantum efficiency of fluorescence, which is emitted by conversion of singlet excitation energy into light emission, is 25%, while phosphorescence, which is emitted by conversion of triplet excitation energy into light emission, can have an internal quantum efficiency of 100% when energy transfer via intersystem crossing from a singlet excited level is taken into account. In view of the above, an organic EL element (a phosphorescent light-emitting element) in which a phosphorescent material is used as a light-emitting substance is selected in many cases so that light is emitted efficiently.
Most of substances capable of efficiently converting triplet excitation energy into light emission are organometallic complexes, and in most cases, central metals of the organometallic complexes are rare metals whose production is small. The price of rare metals is high and greatly fluctuates, and supply thereof might be unstable depending on the global situation. For this reason, there are some concerns about cost and supply regarding phosphorescent light-emitting elements.
To cause conversion of triplet excitation energy into light emission, delayed fluorescence can also be utilized. In this case, not phosphorescence but fluorescence is obtained because reverse intersystem crossing from a triplet excited level of the triplet excitation energy to a singlet excited level of the triplet excitation energy is utilized and the light emission occurs from a singlet excited level. This is readily caused when an energy difference between a singlet excited level and a triplet excited level is small. Emission efficiency exceeding the theoretical limit of emission efficiency of fluorescence has been actually reported.
It has been also reported that a high-efficiency light-emitting element was obtained in such a manner that an exciplex (excited complex) was formed by two kinds of substances to produce a state where an energy difference between a singlet excited level and a triplet excited level is small, and delayed fluorescence of the exciplex was utilized.